1. Field of the Invention
This invention relates to silver wires for use in video display apparatus such as VTR or TV or in audio equipment.
2. Background Information
Conductors conventionally used in recording, acoustic or image transmission applications are made of annealed copper wires, which are produced by cold working oxygen-free or tough pitch copper with a purity of about 99.9 wt % followed by annealing, or copper wires prepared by plating such annealed copper wires.
With copper wires having purities on the order of 99.9 wt %, signals cannot be transmitted correctly without phase differences and, as a result, only blurred images or unsharp sounds are produced. To solve these problems, high-purity copper wires produced by working raw materials having purities of at least 99.999 wt % have recently been introduced into the market.
Similar effects are exhibited by silver wires that are produced by a process which comprises solidifying silver with a purity of at least 99.95 wt % in a longitudinal direction to yield an ingot, drawing it by either cold or warm working and further working the wire under conditions that will not cause recrystallization. This process is described in Unexamined Published Japanese Patent Application No.163505/1986 under the title of xe2x80x9cA Process for Producing Conductors for Use in Video Display Apparatus or Audio Equipmentxe2x80x9d, which is a priority application of Sawada U.S. Pat. No. 4,976,792. However, the demand of the market has solely been for copper wires.
Thus, in the metal purifying technology, 6N and 7N copper materials have successfully been developed (purities of at least 99.9999 wt % are designated herein as xe2x80x9c6Nxe2x80x9d and purities of at least 99.99999 wt % are designated herein as xe2x80x9c7Nxe2x80x9d). On the other hand, practically feasible methods of purifying silver materials were not heretofore developed and silver wires of such high purity have not heretofore been produced that can be tested for their performance to thereby obtain data that compare with high-purity copper wires.
High purity metals prescribed in the Japan Industrial Standard (JIS) are limited to metals with a purity of 99.99% (4N) or less. As regards metals with a purity of more than 99.99%, the grading is made by the manufacturers of the respective metals on their own terms. Accordingly, if a metal with a purity of 99.999% (5N) or a metal with a purity of 99.9999% (6N) is available from a certain manufacturer, the nominal purity is a purity that has been evaluated by the manufacturer itself based on the technical level of analysis of that manufacturer.
The specification of a silver bullion with a purity of 99.9% (3N) and one with a purity of 99.99% (4N) are given in JIS H 2141 (1964). The particulars of the silver ingot compositions set forth on page 346 of JIS H 2141 are as set forth hereinbelow.
Other than referring to JIS H 1181-Methods for Chemical Analysis of Silver Ingot and JIS H 1183-Method for Spectro Chemical Analysis of Silver Ingot, there is no other mention on page 346 of JIS H2141 as to what kind of analytical technique is to be used. There is also no mention on page 346 of JIS H 2141 as to whether the xe2x80x9c%xe2x80x9d is wt % or atom %. It is considered, however, that xe2x80x9c%xe2x80x9d means xe2x80x9cwt %xe2x80x9d on page 346 of JIS H 2141.
High purity silver was described in a catalogue dated Jan. 15, 1994 from Johnson Matthey Company of Spokane, Wash., a world-famous manufacturer of high purity metals. The Johnson Matthey Company sells a variety of high purity metals in a relatively small amount. The Jan. 15, 1994 Johnson Matthey catalogue provided the following typical analysis (ppmw) for a grade of silver:
Based on the above analysis, it may therefore have been possible to obtain high purity silver with a purity that some may have considered to be of xe2x80x9c99.9999% or morexe2x80x9d by making use of a xe2x80x9czone-melting processxe2x80x9d, or the like. However, the meaning of xe2x80x9c99.9999% (6N)xe2x80x9d in conjunction with the Jan. 15, 1994 Johnson Matthey catalogue is not the same as the term xe2x80x9c99.9999% (6N)xe2x80x9d with respect to the present invention.
In the case of the Jan. 15, 1994 Johnson Matthey catalogue, the designated impurities are limited to elements inclusive of Al, Ca, Cu, Fe, Mg, Pb and Si. It is stated in the Johnson Matthey catalogue that the typical analysis is by emission spectroscopy, and that glow discharge mass spectroscopy (GDMS) is available on a lot by lot basis on special request. It is also stated in the Johnson Matthey catalogue that the numerical values of the impurities are given in terms of ppmw. Thus the accuracy of the typical analysis set forth in the Jan. 15, 1994 Johnson Matthey catalogue cannot be fully determined.
In the xe2x80x9cHandbook of Precious Metalsxe2x80x9d, p. 529 (1989), the footnote at the bottom of Table A.4 described a Ag content of 99.9999% and more.
It is not known what kinds of elements were analyzed as impurities in silver with a Ag content 99.9999% or more as described in the footnote to said Table A.4. Assuming that the same elements as set forth in the Table A.4 are analyzed as the designated impurities in the silver with a Ag content 99.9999% or more (Table A.4 lists the following impurities: Au, Pt, Pd, Fe, Pb, Bi, Sb, Te and Zn), such impurities are not the same as all the elements which were analyzed as designated impurities in the 6Nxe2x80x94Ag of the present application. Also, there is no mention in the xe2x80x9cHandbook of Precious Metalsxe2x80x9d as to which method was used for the analysis of impurities in silver.
An object, therefore, of the invention is to establish a process for producing high-purity silver materials by novel means and thereafter work them into wires, thereby providing high-purity silver wires suitable for use in recording, acoustic or image transmission applications.
The present inventors conducted intensive studies with a view to attaining the stated object and found that by using a special vacuum distillation refiner, high-purity silver with a purity of at least 99.9999 wt % could be obtained that had a total impurity content of less than 1 ppmw as measured by glow discharge mass spectrometric analysis. When high-purity silver wires drawn from this raw material were assembled into video display apparatus or audio equipment, there could be produced images or sounds that were sharper than those obtained with the conventional high-purity copper wires. The present invention has been accomplished on the basis of this finding.
Thus, the stated object of the invention can be attained by a high-purity silver wire for use in recording, acoustic or image transmission applications, which contains sulfur, iron, copper, palladium, gold and lead in such amounts that none of them will exceed 0.5 ppmw (ppm wt.) as they are taken individually and wherein any other impurities that can be measured in an amount not less than the respective lower limits of detection add up to less than 1 ppmw.
The present invention also provides a high purity silver wire for use in recording, acoustic or image transmission applications, wherein the high purity silver wire consists of silver having a purity of not less than 99.9999 wt % that is determined by subtracting a total amount of the contents of specified impurities given in wt % from the numeral 100 given in wt %, and wherein only the measurements exceeding the detection limits of the respective elements are added to obtain said total amount, the specified impurities consisting of the predetermined twelve elements inclusive of Na, Si, S, K, Ca, Cr, Fe, Ni, Cu, Pd, Pb and Au, the contents of the respective elements being determined by glow discharge mass spectrography whose detection limit for Au is 0.1 ppmw and whose detection limit for each of the other eleven elements is 0.01 ppmw.